Method of conditioning aluminous surfaces for the reception of electroless nickel plating

ABSTRACT

ALUMINUM AND ALUMINUM ALLOYS ARE SUBJECTED TO A PREPLATING TREATMENT WHICH CONDITIONS THE SURFACES OF THESE METALS FOR RECEIVING ELECTROLESSLY DEPOSITED NICKEL PLATING DIRECTLY THEREON. A CLEAN ALUMINOUS SURFACE IS PREPARED FOR ELECTROLESS PLATING BY ETCHING IN AN ALKALINE SOLUTION AND THEN PICKLING IN AN ACIDIC SOLUTION CONTAINING CHLORIDE IONS. AN ALKALINE SOLUTION CONTAINING HYPOPHOSPHITE IONS IS EMPLOYED TO ACTIVATE THE ALUMINOUS SURFACE WHICH IS THEN PROVIDED WITH A THIN ELECTROLESS NICKEL STRIKE COAT BY IMMERSION IN AN AMMONICAL SOLUTION CONTAINING NICKEL IONS, HYPOPHOSPHITE IONS, AND A CHELATING AGENT. AFTER THE ELCTROLESS STRIKE COAT IS APPLIED, THE ALUMINOUS SURFACE IS PROVIDED WITH A PLATING OF ELECTRLESS NICKEL IN A CONVENTIONAL, ESSENTALLY HALOGEN-FREE BATH.

United States Patent f U.S. Cl. 11750 3 Claims ABSTRACT OF THE DISCLOSURE Aluminum and aluminum alloys are subjected to a preplating treatment which conditions the surfaces of these metals for receiving electrolessly deposited nickel plating directly thereon. A clean aluminous surface is prepared for electroless plating by etching in an alkaline solution and then pickling in an acidic solution containing chloride ions. An alkaline solution containing hypophosphite ions is employed to activate the aluminous surface which is then provided with a thin electroless nickel strike coat by immersion in an ammonical solution containing nickel ions, hypophosphite ions, and a chelating agent. After the electroless strike coat is applied, the aluminous surface is provided with a plating of electroless nickel in a conventional, essentially halogen-free bath.

The present invention relates generally to a pre-plating treatment for aluminum and its alloys, and more particularly to an improved method of conditioning such metal for subsequently applied electroless nickel plate. This invention was made in the course of, or under, a contract with the U.S. Atomic Energy Commission.

The corrosion resistance of aluminum and aluminum alloys is generally sufiicient to permit the use of such metals in many applications without providing additional protection to exposed surfaces. However, in some applications it may be preferable to provide protective coatings on aluminous surfaces when the particular atmosphere or other environmental media are corrosive to the aluminum or aluminum alloy being used. Historically, the plating of aluminum and aluminum-containing alloys has been somewhat diflicult and unreliable due to the presence of its relatively impervious and rapidly formed natural oxide film or reactions between the alloying ingredients and environmental conditions. If this surface film is not removed or the effectiveness thereof reduced, the plating only mechanically adheres to the film-covered portion of the surface, which eventually results in a plated surface that peels, blisters, or cracks, particularly when the surface is subjected to strains such as produced under the influence of elevated temperatures, flexing, and bending. In addition to the problems associated with the aforementioned Efilm, techniques of electrolessly depositing nickel onto aluminous surfaces have not been particularly successful since the aluminous surfaces are not normally receptive to nickel plate. Further diificulties arise when preparing aluminous surfaces for electroless nickel since each aluminum alloy requires a different or modified surface preparing technique.

In order to overcome the above and other problems associated with electroless nickel plating of aluminum and aluminum alloys, many surface conditioning and plating procedures have been attempted. Of the various procedures previously practiced some satisfaction has been achieved by providing clean and etched aluminous surfaces with a base coat of Zinc or tin by immersion deposition and then covering the base coat with a layer of electro- 3,666,529 Patented May 30, 1972 deposited copper. After electrolessly depositing a nickel plate on the coated aluminum, the plated structure is often heated to form a bi-metal diffusion layer between the metals and thereby enhance the bond. In addition to this procedure being a somewhat cumbersome and complex procedure, the heat treatment of the plated aluminum alloy to enhance the bond often seriously impaired the strength of the aluminum alloy.

It is the aim of the present invention to provide a new and improved method of conditioning aluminum and its alloys prior to electroless nickel plating by employing a pre-plating or surface conditioning treatment useful for aluminum and aluminum alloys without variations in the pre-plating treatment such as heretofore required for the different alloys. The present invention also obviates the use of the previously required pre-coatings of different metals in order to obtain adherent electroless nickel plate. Aluminous surfaces are conditioned prior to immersion into an electroless nickel plating solution by contacting a clean and degreased surface portion with an alkaline etch and then contacting the aluminous surface with an acidic solution containing chloride ions which expose the aluminum crystals. The aluminous surface is then activated in an alkaline solution containing hypophosphite ions and thereafter provided with a thin strike coat of amorphous nickel to maintain the activated or catalytic condition of the surface for facilitating the reception and bonding of the subsequently deposited electroless nickel plate.

An object of the present invention is to provide a new and improved method of conditioning aluminous surfaces for the reception of electrolessly deposited nickel.

Another object of the present invention is to provide a method of pre-treating the surface of aluminum and aluminum-containing alloys prior to electroless nickel plating whereby the treatment conditions the surface in such a manner as to obviate both the use of a different pre-treating solution for each alloy and pre-coatings of aluminous surfaces with different metals for establishing an adequate bond with the nickel.

Other and further objects of the invention will be obvious upon an understanding of the illustrative method about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Described generally, the present invention provides a unique process for conditioning aluminous surfaces whereby nickel may be electrolessly plated directly onto aluminum and any of its alloys regardless of the alloying elements. For example, aluminum alloys with alloying elements such as set forth in the following table have been satisfactorily treated in accordance with the teachings of the present invention.

TABLE Aluminum Association Alloy No. Aluminum-99.00% minimum and greater lXXX Major alloying element Aluminum alloys grouped by major alloying elements:

Examples of alloys in the above table which have been surface treated for receiving electroless nickel plate include the alloys with the Aluminum Association designation numbers 1100, 2024, 3003, 5052, 6061, and 7075.

The aluminum and aluminum alloys are conditioned preparatory to receiving the electrolessly applied nickel by contacting a particular aluminous surface with an alkaline etchant and then with an acidic solution containing nickel chloride wherein the surface is etched by the chloride ions to expose the aluminum crystals. The electrochemically deposited nickel coating resulting from the etching operation is removed by contacting the coating with a concentrated nitric acid solution. The aluminous surface is activated by being immersed in an ammoniacal solution containing an adequate quantity of hypohosphite ions and thereafter contacted with an aqueous solution containing nickel ions, hypophosphite ions, and a chelating agent at a temperature in the range of about 3590 C. and a pH in the range of about 9-9.5 for coating the activated or catalytic surface with a thin, electroless strike coat of amorphous nickel. After conditioning the aluminous surface as above described, the latter may be provided with an electrolessly deposited nickel plate of any desired thickness by employing a conventional, essentially halogen-free electroless plating solution.

Coupons of aluminum and aluminum alloys are initially cleansed of grease and other easily removed foreign matter on the surface by a conventional cleaning procedure such as by employing a scouring powder together with a degreasing agent such as chromic acid. After rinsing the cleaned aluminous surfaces of the coupons in water, the surfaces are etched for a suitable duration of about one minute in a dilute or weak, e.g., percent, sodium or potassium hydroxide solution for dissolving surface oxides and other impurities. The coupon is then rinsed with water and etched and pickled in an acidic solution containing chloride ions for a duration of about 0.5 to 2 minutes with the solution at room temperature. The chloride ions attack the aluminous surface in such a manner as to expose the aluminum crystals for facilitating the reception of the subsequently applied nickel plate. Satisfactory results have been achieved by employing a solution with a chloride ion contributor such as nickel chloride hexahydrate and a carboxylic acid such as. lactic acid; or, if desired, nitric acid may be substituted for the carboxylic acid. The concentration of the solution may be about 175 to 700 grams per liter of nickel chloride hexahydrate and to 750 milliliters per liter of concentrated (85 percent) lactic acid or 300 to 500 milliliters per liter of 71 percent nitric acid with the balance being a saturated solution of NiCl When using the lactic acidnickel chloride solution, a thin film of nickel is electrochemically deposited on the surface, which is readily removed or deplated by dipping the coupon in a concentrated (16 N) nitric acid solution. When nitric acid is substituted for the carboxylic acid, the electrochemically deposited nickel is immediately redissolved in the same solution.

The deplated coupon may be desmutted in a suitable acidic solution such as a nitric acid solution containing a suflicient quantity of hydrofluoric acid to remove silicon and other surface impurities; for example, a solution with 2 milliliters per liter of 48 percent hydrofluoric acid is satisfactory. After rinsing in Water, it may be preferable to repeat the aforementioned treatment, beginning with a -second sodium hydroxide etch to assure that the aluminous surface is adequately pickled and etched.

The surface of the coupon is activated or placed in a catalytic condition by immersing the coupon in a solution consisting essentially of a hypophosphite ion contributor such as sodium or potassium hypophosphite and a Weak alkali such as ammonium hydroxide. The concentration of the solution may vary in the range of 5 to 100 grams of the hypophosphite ion contributor per liter and 5 to 50 milliliters of ammonium hydroxide per liter. With other weak alkalis the solution should be correspondingly basic. The time required for the activation is normally about 1 to 5 minutes at room temperature in solutions in the aforementioned concentration ranges.

Immediately after the surface of the coupon is activated, the coupon is placed in a basic solution containing hypophosphite anions and nickel cations and a chelating agent for providing the latter with a thin electroless strike coat or layer of nickel of a thickness of about 0.03 of a mil. This nickel is amorphous and thereby greatly enhances the bond between the subsequently applied electroless nickel and the coupon. The solutions found suitable for providing the strike coat are solutions containing 5-75 grams per liter of sodium or potassium hypophosphite and 5-60 grams per liter of nickel sulfate. A chelating agent is then added to the hypophosphite and nickel cation solution. The chelating agent may be any suitable compound such as ammonium citrate or any of the aminopolycarboxylic acids and alkaline earth metal salts thereof. The quantity of the chelating agent should be such that it will complex or tie up substantially all the nickel ions in the plating solution. The pH of the preplating solution is preferably maintained between 9 and 9.5 and is readily adjusted to be contained in this range by employing ammonium hydroxide or the like. The temperature of the bath is maintained in a range of -90 C. to provide the electroless nickel strike coat after an immersion period of about one minute.

After the coupon has been provided with the aforementioned nickel strike coat, it is preferably dipped into an acidic, electroless nickel plating bath that is preferably substantially free of halogens since the latter form a film on the surface and prevent desirable bonding be tween the nickel plate and the aluminum. This immersion in the bath for approximately 15 seconds cleans the part of free ammonium and nitrate ions.

In order to plate the coupon when treated by the novel pretreatment steps described above the coupon is placed in a final electroless nickel plating bath (again one essentially free of halogens for the above reasons), preferably one with a high plating rate. The electroless nickel plating bath may be any suitable commercially available acid-type bath such as an aqueous nickel sulfate bath in which nickel ions are chemically reduced in an aqueous hypophosphite solution. The nickel plate may be of any desired thickness depending upon the particular use envisioned for the nickel-plated aluminum or aluminum alloy. The aluminum and aluminum alloys electrolessly nickel plated when treated as aforementioned do not require any modification of the above procedure regardless of the aluminum or aluminum alloy being treated. The quality of the bond between the nickel and the aluminum surface was evaluated by cross sectioning, bending, and heating to 450 C. and immediately thereafter quenching in cold water. These tests showed excellent adherence of the nickel coating to all alloys.

In order to provide a more facile understanding of the present invention, a typical aluminous surface pretreating or conditioning operation is set forth below. The coupon employed in this example is an aluminum alloy with the Aluminum Association numerical designation 5052-H32.

EXAMPLE The aluminum alloy coupon is prepared for receiving an electroless nickel plate by cleaning it 'with scouring powder, rinsing it thoroughly in Water, degreasing it with chromic acid, and again rinsing it thoroughly with Water. The coupon is then etched for one minute in a 5 percent sodium hydroxide solution, rinsed thoroughly with water, pickled one minute at room temperature in a solution of 640 grams per liter of nickel chloride hexahydrate and milliliters per liter of 85 percent lactic acid, and rinsed with Water. The coupon is dipped in concentrated nitric acid to deplete the electrochemically deposited nickel, dipped in an 8 N nitric acid solution which contains 2 milliliters per liter of 48 percent hydrofluoric acid to desmut the coupon surface, and rinsed with water. The above steps, beginning with the etching step, are repeated to assure adequate surface preparation. The coupon is then placed in a solution containing 25 grams of sodium hypophosphite per liter and 25 milliliters of ammonium hydroxide per liter for two minutes at room temperature to activate the surface. The coupon is then placed in the activation electroless nickel strike bath for approximately one minute at 85 C. to 90 C. The coupon is completely covered with a thin (0.03 of a mil) layer of electroless deposited nickel. The solution was composed of the following:

25 g./liter sodium hypophosphite (NZ-HzPO'zHzO) 25 g./liter nickel sulfate (NiSO )-6H O 50 g./liter ammonium citrate [(NH HC H O The pH of the solution is adjusted with ammonium hydroxide to remain between 9.0 and 9.5. The coupon is dipped in a preliminary acid electroless nickel plating bath (containing essentially no halogens) for approximately 15 seconds to cleanse the part of the ammonium and nitrate ions. The coupon is placed into the final conventional electroless nickel plating bath (containing essentially no halogens) to produce a desired thickness of nickel coating on the surface of the aluminum alloy coupon.

It will be seen that the surface conditioning method of the present invention sets forth a singificant advancement in the art of electroless plating aluminum with nickel. The quality of the nickel deposition does not vary between the various aluminum alloys, and the bond between the nickel and aluminum is excellent in the asplated condition and therefore does not require the heat treatment as previously considered necessary to effect the diffusion bond between the nickel plate and the aluminous surface. The elimination of this heat treatment step is significant in that the use of such heating often results in a significant loss in the strength of the aluminum such as in the case of the alloy with the numerical designation 7075.

What is claimed is:

1. In the art of nickel plating aluminum and aluminum alloys by the sequential steps of alkaline etching through plating with an acidic, essentially halogen-free electroless nickel plating solution, an improved method of conditioning an alkaline etched aluminous surface preparatory to receiving the electrolessly applied nickel, comprising the steps of contacting the aluminous surface with an acidic pickling solution consisting essentially of nickel chloride hexahydrate in a concentration of 175 to 700 grams per liter of solution and concentrated lactic acid in a concentration of about 10 to 750 milliliters per liter of solu tion with the contacting being for a duration of about 0.5 to 2 minutes to pickle the surface and electrochemically deposit a coating of nickel thereon, removing the coating of nickel by contacting the coating with a nitric acid solution, contacting the surface with an acidic solution containing sufficient nitric acid and hydrofluoric acid to desmut the surface, activating the aluminous surface by contacting the latter with an alkaline solution containing an adequate quantity of hypophosphite ions, and contacting the surface with a basic solution containing nickel ions, hypophosphite ions, and a chelating agent at a temperature in the range of about C. to C. and a pH in the range of about 9.0 to 9.5 for coating the activated surface with a strike coat of amorphous catalytic nickel, thereafter immersing the aluminum surface into the acidic electroless nickel plating solution to effect the formation of the nickel plate on the catalytic nickel strike coat.

2. The improved method of conditioning an aluminous surface as claimed in claim 1, wherein the alkaline solution consists essentially of about 5 to grams of hypophosphite ion per liter and about 5 to 50 milliliters of ammonium hydroxide per liter, and wherein the duration of contact between the aluminous surface and the alkaline solution is about 1 to 5 minutes with the solution at about room temperature.

3. The improved method of conditioning an aluminous surface as claimed in claim 1, wherein the basic solution consists essentially of about 5 to 75 grams of the hypophosphite ion per liter, about 5 to 60' grams nickel sulfate per liter, an adequate quantity of the chelating agent to complex substantially all the nickel ions in the basic solution, and an adequate quantity of an alkali to maintain the pH of the basic solution in said range.

References Cited UNITED STATES PATENTS 1,011,203 12/1911 Jahn 156-22 2,532,283 5/1947 Brenner et al. 117-50 2,958,610 11/1960 Ramirez et al. 117-50 3,032,443 5/1962 Shout 117-160 3,329,522 7/ 1967 Saubestre et al. 117-50 OTHER REFERENCES Metal Finishing (Productivity Report), N.Y., Anglo- American Council on Productivity, 1950, p. 19.

Condensed Chemical Dictionary, 6th ed., N.Y., Reinhold, 1961, p. 795.

Gebauer, K., Hard Chrome Plating on Aluminum Alloys, in Metal Finishing, July 1950, pp. 56 and 57.

ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner US. Cl. X.R. 

